1. Field of the Invention
The present invention relates to a drive circuit for a light emitting device which emits light by current injection, and more particularly, to a drive circuit for an organic electroluminescence device (hereinafter, referred to as organic EL device).
2. Description of the Related Art
U.S. Pat. No. 6,373,454 describes an active matrix type display apparatus including a drive circuit for a light emitting device, which includes drive transistors for controlling light emission of each pixel by a signal line corresponding to a column of display pixels and a scanning line corresponding to a row of display pixels. U.S. Pat. No. 6,373,454 describes a configuration of a write current drive circuit capable of reducing variations in characteristics of the drive transistors.
FIG. 12 illustrates a configuration example of a drive circuit. The drive circuit of FIG. 12 includes a light emitting device EL, switches M1 to M3 formed of n-type transistors, a drive transistor M4 for driving the p-type light emitting device EL, and a storage capacitor C1. The drive circuit is operated by a feeder PVdd, a signal line DA, and scanning lines P1 and P2.
FIG. 13 is a time chart of the scanning lines P1 and P2. During a period from t1 to t2, the switches M1 and M2 are turned on while the switch M3 is turned off, whereby the drive circuit performs a writing operation. During this period, a data voltage indicating display luminance, which is supplied from the signal line DA, and a threshold voltage of the drive transistor M4, are written in the storage capacitor C1. Next, during a period from t2 to t3, the switches M1 and M2 are turned off while the switch M3 is turned on. Accordingly, a current corresponding to the data voltage written in the storage capacitor C1 is supplied to the light emitting device from the feeder PVdd, and the light emitting device EL illuminates. Subsequently, during a period from t3 to t4, the switches M1 and M2 are again turned on while the switch M3 is again turned off, whereby the light emitting device EL is turned off. Through this operation, it is possible to reduce variations in luminance among respective pixels, which are caused by variations in threshold voltages of the drive transistors M4 provided to the respective pixels.
FIG. 1 and FIG. 2 illustrate, as an example of the light emitting device, a relationship between driving time and luminance, and a relationship between driving time and voltage, respectively, when the organic EL device is driven with a constant current. As can be seen from FIG. 1 and FIG. 2, when the current is supplied to cause the organic EL device to emit light, there occurs a deterioration phenomenon of a device, such as a decrease in emission intensity (luminance) or a rise in voltage with a lapse of the driving time. A degree of deterioration of the device differs among the respective pixels of the active matrix display apparatus, and the deterioration of the organic EL devices of the respective pixels occurs as a burn-in phenomenon in a display region of the display apparatus. This burn-in phenomenon is recognizable even when there is a small range of variations in luminance, such as variation of approximately 2% between adjacent pixels.
In order to deal with the above-mentioned problem, Japanese Patent Application Laid-Open No. 2006-91709 describes a display apparatus which detects a voltage between terminals of each organic EL device arranged in each pixel when light is emitted with luminance corresponding to image data, and compensates for a decrease in luminance of the each device according to an amount of rise in voltage between the terminals of the device, which is caused due to deterioration of the device.
However, it is required for the display apparatus according to Japanese Patent Application Laid-Open No. 2006-91709 to be provided with a table for holding a correction coefficient to compensate for a decrease in luminance of the device or a multiplier circuit for multiplying image data by the correction coefficient outside a display panel in which pixels are arrayed. As a consequence, cost for the display apparatus increases, which is a serious problem for a small display apparatus for which cost reduction is required.